Move file hashing to a worker thread, add async method

[stroncium]

Move file hashing to thread, add async method, fixes #16

[stroncium]

@sindresorhus This one is ready, as far as I can tell.

[sindresorhus]

You need to add thread.js to files in package.json.

[sindresorhus]

Can you document in the method that uses worker threads that they use worker threads and are async?

[sindresorhus]

Also note sindresorhus/crypto-hash@46f2f28

[stroncium]

@sindresorhus Style fixed, added notes regarding worker_threads to relevant methods in ts and readme.

What do you think of moving worker_threads functionality into microlib in the future? We have partially mirroring implementation both in hasha and crypto-hash now(more functionality in hasha). I think have enough info to unify it.

[sindresorhus]

What do you think of moving worker_threads functionality into microlib in the future? We have partially mirroring implementation both in hasha and crypto-hash now(more functionality in hasha). I think have enough info to unify it.

Yeah, that would be great. I have a note somewhere on my computer about asking you to do that, actually.

[sindresorhus]

Looks good :)